As is widely known in the imaging arts, a thermographic imaging process relies on the use of heat to help produce an image. Typically, a thermally-sensitive image-forming layer is coated on top of a suitable base or substrate material such as paper, plastics, metals, glass, and the like. The resulting thermographic construction is then heated at an elevated temperature, typically in the range of about 60.degree.-225.degree. C., resulting in the formation of an image. Many times, the thermographic construction is brought into contact with the thermal head of a thermographic recording apparatus, such as a thermal printer, thermal facsimile, and the like. In such instances, an anti-stick layer is coated on top of the imaging layer in order to prevent sticking of the thermographic construction to the thermal head of the apparatus utilized.
Thermographic materials whose image-forming layers are based on silver salts of long chain fatty acids, such as silver behenate, are known. At elevated temperatures, silver behenate is reduced by a reducing agent for silver ion such as hydroquinone, substituted hydroquinones, hindered phenols, catechol, pyrogallol, methyl gallate, leuco dyes, and the like, whereby an image is formed.
It is also known that other additives can be added to imaging layers of a thermographic construction to enhance their effectiveness. For example, U.S. Pat. No. 2,910,377 discloses that the silver image for such materials can be improved in color and density by the addition of toners to the imaging layer. Toners which give primarily image density enhancement are also referred to as development accelerators.
Thermographic elements are typically imaged with the use of a thermal printhead whereby heated styli are pressed into intimate contact with the thermographic element or media. When electrically pulsed, the styli are heated which in turn heats the thermographic media which contains two or more components which combine and produce a legible, colored mark. The resulting image is built up in a spotwise manner. To provide good images without voids and with uniform image areas, the surface of the thermal imaging media needs to have good thermal printhead matching characteristics. A good media will have characteristics including maximized slip (i.e., the ease of transport of media underneath the printhead) and minimized "pick-off" (i.e., the removal of topcoat adhering as residue to the printhead which results in image voids).
Conventional thermal printing media or thermographic elements achieve their thermal printhead-matching characteristics typically using high loading of fillers and pigments such as silica, calcium carbonate, clay, and the like. The use of such conventional anti-stick agents in a topcoat on the thermographic element contributes to haze and greatly diminishes the usefulness of the thermographic element for overlaying, projection, or applications where it is used as a mask.
Additionally, during the thermal printing process the pressure and high temperature that the media are exposed to distort the surface of the thermographic element. This thermal marring makes it difficult to achieve high gloss of the resulting images. The high matting effect of higher filler loading gives low gloss media and tends to blend-in low gloss images.
In view of the foregoing, new and improved anti-stick topcoats for thermographic media imaged by thermographic recording apparatus are needed in the industry.